The present invention relates to calcining gypsum while in suspension in a heated gas stream. More particularly, the invention relates to a process and apparatus for calcining gypsum while in suspension in a hot gas stream which permits a control over the degree of calcination to be achieved.
The raw material for gypsum is calcium, sulfate, dehydrate (CaSO.sub.4 2H.sub.2 O) which can be converted by calcining to a number of phases which depend on the calcining conditions. The raw material is dehydrated at approximately 375.degree. F. to form the hemihydrate (2CaSO.sub.4 H.sub.2 O) which is basis of most gypsum plasters and called calcined gypsum or, when used for making ornaments or casts, it is called Plaster of Paris. When mixed with water it forms the hydrated sulfate that solidifies and sets firm due to interlocking crystallization.
If calcined further to anhydrous anhydrite (CaSO.sub.4) the material may be used as a paper filler under the name of Pearl filler. An anhydrous calcium sulfate in powder or granular form will absorb 12% to 14% of its weight of water and is used as a drying agent for gases and chemicals. It can be regenerated for reuse by heating the material and drying off the water.
Prior to the present invention, most plaster manufacturing has been done in a batch calcining kettle which accomplishes the work in 21/2 hours at temperatures on the order of 280.degree. F. to 335.degree. F. in a 15 ton batch. Feed size is around 11/4 inch as the largest size. The kettle product is ground to produce a final product. Plaster has also been manufactured in a rotary kiln utilizing a stone feed on the order of 11/2 inch size. The product goes through a hammer mill followed by a grinding mill to produce 70% to 95% minus 100 mesh plaster product. Neither the rotary kiln system nor the batch calcining kettle of common practice has much heat recovery and is not an energy efficient process.
Calcining gypsum in a current of hot gas is disclosed in U.S. Pat. No. 3,489,994 issued Mar. 14, 1972. This patent discloses calcining the gypsum while the material is within suspension in a hot gas stream in a cyclone. There may be two stages of heating with hot gas serially supplied first to one cyclone then to a second cyclone. The material to be calcined is supplied first to the second cyclone and then alternately either to the first cyclone or to a cooling apparatus. Water is sprayed onto the calcined product for cooling the product and controlling moisture content. With this prior device, the moisture content in the product is controlled by the cooling cycle. Thermal economies are not achieved with this type of apparatus because the heat content in the product is not utilized in the calcining process. In addition, there is no arrangement for controlling the degree of calcination within the calcining vessel.
Another type of apparatus for calcining gypsum in suspension is disclosed in U.S. Pat. No. 3,956,456 issued May 11, 1976. In this device a complex calcining furnace is utilized and there is an absence of thermal economy in the system.
It would be desirable to provide a process and apparatus for calcining gypsum which utilizes the heat of the product in the calcining process and the spent calcining gases in a preheating stage in order to reduce fuel consumption of the total process.
Prior practice has relied upon calcining temperature to vary the degree of calcination. According to the present invention, material can be recirculated through the calcining furnace to control the amount of calcination. At varied temperatures and degree of calcination different products of gypsum can be produced, for example 320.degree. F. product is used for modeling plaster and at 650.degree. F. gypsum plaster is produced.